Aqueous composition containing a semiconductor

ABSTRACT

The use of an aqueous composition comprising a semiconductor as cleaning agent for surfaces in the outdoors field and indoors field is proposed.

[0001] The invention relates to the use of an aqueous compositioncomprising a semiconductor as a cleaning agent for surfaces.

[0002] In many areas, the unwanted biological colonization of surfaceswith bacteria, yeasts, algae, mosses, lichens etc. plays an importantpart, which can give rise to health hazards, corrosion, danger ofslipping and unsightliness.

[0003] This relates in particular to textiles, household surfaces, forexample working surfaces in the kitchen, applications in the sanitationsector, building facades, floor coverings or other surfaces which areexposed to weather or organic impurities such as exhaust gases. In allof these sectors, that is to say in the indoors and outdoors sectorsequally, it is desirable or even necessary to clean certain surfacesregularly or keep them low in microbes or even microbe-free.

[0004] It is known for this purpose to keep open surfaces clean bymechanical cleaning using bactericidal agents.

[0005] The disadvantage with the known cleaning processes is thatinfection or fouling can start again immediately after the cleaningoperation. The bactericidal action depends on the care of the respectivecleaning operative and the cleaning intervals. A high microbe-free or“clean” state can therefore only be maintained laboriously and withgreat effort. In addition, the problem is made more difficult by thefact that some bacteria or microbes develop a certain resistance towardthe bactericidal agents, so that disinfection, despite careful work,does not in fact occur, or occurs only to an insufficient extent. Undersome circumstances, this error is nor even noticed.

[0006] Extensive disinfection measures using formaldehyde or ethyleneoxide, for example at quarantine stations, represent great problems fromthe toxicological aspect, since these gases penetrate the material andhave relatively long outgassing times. As a result, a larger supply ofinstruments or rooms must be kept ready, since the availability isrestricted by these times.

[0007] The photo-induced disinfecting activity of semiconductormaterials, in particular titanium dioxide, is known and is described,for example, in Blake D. M. et al. in Sep. Pur. Meth. 28(1999), pages 1to 50.

[0008] DE-A 196 54 109 discloses designing the articles to bedisinfected to have a surface layer which comprises a semiconductormaterial and into which is coupled UV radiation from a UV radiationsource. As a result, the surface layer is disinfected or acts in anoxidizing manner. However, such a solution has the disadvantage that thearticles to be treated must be fixed to a surface layer which comprisesa semiconductor material.

[0009] In contrast, it is an object of the invention to provide acleaning process for surfaces which is easy to handle, makes possibleindividual and targeted treatment of the sites which are to bedisinfected, using a cleaning agent, which, when required, can beremoved again without problem and specifically.

[0010] In particular, it is an object of the invention to provide acleaning agent having significantly improved activity compared withknown cleaning agents.

[0011] According to the invention this object is achieved by using anaqueous composition comprising a semiconductor at a concentration offrom 5 to 50 mg, based on 1 l of aqueous composition, as cleaning agentfor surfaces, under the action of light.

[0012] Thus, according to the invention, aqueous compositions are usedwhich comprise a semiconductor at high dilution. It has been found that,owing to the high dilution of the semiconductor, no continuous layersform on the surfaces to be treated. This leads to a type ofdiscontinuous application of the semiconductor. It has surprisingly beenfound that this is essential for good activity of the photocatalyst.Although the mechanism of action has not yet been completely explained,it is assumed that owing to the fact that the semiconductor is presentin particles which are isolated from one another, the electron holes andelectrons formed by the photocatalytic process cannot migrate throughthe semiconductor and therefore are more available for reaction comparedwith organic substances or microorganisms. In contrast, in the case of acontinuous, extended semiconductor layer, as a result of the migrationof electrons and electron holes, these are no longer available forreactions. Owing to the fact that the electrons and electron holes canbe distributed over broad sections in the semiconductor layer, theprobability that they are available for reactions at the surface of thesemiconductor is less. Consequently, a markedly poorer activity ofdisinfection or cleaning results. Furthermore, these processes couldlead to the fact that the probability of recombination of electrons andelectron holes increases. This would be the reverse of the effect whichwas started by the action of light, likewise with the consequence ofdecreased activity.

[0013] Particularly preferably an aqueous composition is used whichcomprises a semiconductor at a concentration of from 10 to 50 mg, basedon 1 l of aqueous composition.

[0014] It is preferred in particular that the semiconductor is titaniumdioxide, preferably a titanium dioxide which is at least 70% by weightin the anatase modification.

[0015] It is also possible to use the semiconductor, not in pure form,but in the form of a semiconductor doped with one or more transitionmetals of subgroup 8, in particular a titanium dioxide doped withplatinum and/or rhodium. In this case the doped semiconductor is tocontain at least 60% by weight of the semiconductor ion and less than40% by weight of the doping ion. Those which are particularly active aredoped semiconductors having a regular distribution of the doping ions inthe semiconductor matrix, and are described, for example, in WO99/33564.

[0016] The phototoxic and oxidative action of the semiconductor materialin the aqueous composition is effected by light, that it to say byelectromagnetic radiation, in particular of a wavelength in the rangefrom 350 to 400 nmn, preferably 380 nm, provided that undopedsemiconductor material is used. Thus undoped semiconductor material canbe used preferably in the outdoors field or in the indoors field withthe action of artificial light. In the event that, as described above,doped semiconductors are used, the cleaning and/or disinfection actionis preferably achieved using light of a wavelength in the range from 400to 650 nm. Daylight is sufficient for this. Direct solar irradiation isnot required, and diffuse light, for example in the indoors field, isalso sufficient. Obviously, any artificial light source which gives offradiation in the abovementioned wavelength range can also activate thesemiconductor material. Doped semiconductor material can thus be usedwithout restrictions in the outdoors field, just as in the indoorsfield, with the action of sunlight or diffuse light in the indoorsfield, as also under the action of artificial light in the indoorsfield.

[0017] Semiconductor materials are generally not water-soluble; aqueouscompositions of same can be provided in the form of dispersions byphysical distribution of finely divided semiconductor particles inwater. Such dispersions are used according to the invention for cleaningsurfaces.

[0018] In this case there are no restrictions with respect to thesurfaces to be treated and the impurities to be removed; the use in theoutdoors field is possible for treating buildings or paths thereof, forexample facades or outdoor surrounds of buildings, such as footpathpaving slabs, to combat fouling due to macroscopic organisms, such asalgae, lichens, mosses or slime-forming bacteria, or in the indoorssector for cleaning and disinfecting in the hygienically relevantsectors, for example in hospitals, in food-processing enterprises, inthe beverage, cosmetics or pharmaceuticals industry, in bioengineeringand genetic engineering, in particular against microscopic organisms,such as bacteria, fungi, viruses or amebae.

[0019] The semiconductor material, preferably titanium dioxide, isparticularly active in a mean size of the primary particles, measured bytransmission electron microscopy, in the range from 10 to 2000 nm,preferably in the range from about 20 to 200 nm.

[0020] It is possible to use the aqueous composition comprising asemiconductor without further additives as cleaning agent. However,preferably, one or more of the auxiliaries listed hereinafter can beadded: adhesion promoters, solubilizers, thickeners, surface-activeagents and dispersants, in the amounts customary therefor.

[0021] On the auxiliaries individually:

[0022] adhesion promoters are substances which enhance the adhesion ofthe aqueous composition to surfaces. The semiconductor material is toremain on the surfaces to be treated for a relatively long time. Owingto the low semiconductor concentration, after the application, forexample by spraying, on the surface to be treated, a continuous layerdoes not form. The adhesion promoter ensures that the individualisolated semiconductor particles also remain sufficiently long on thesurface and thus can develop their activity. Preferred adhesionpromoters are short-chain polymers, for example natural and syntheticrubbers, polyacrylates, polyesters, polychloroprenes, polyisobutenes,polyvinyl ethers or polyurethanese. These can also be used incombination with other additives, such as resins, plasticizers and/orantioxidants.

[0023] The solubilizers preferably used act so that substances dissolvein an enhanced manner in a solvent in which they are usually onlyslightly soluble, in the present case customarily sparinglywater-soluble organic substances. Solubilizers which can serve in thepresent case are, for example, organic solvents which arewater-miscible, such as short-chain alcohols, in particular ethanol orisopropanol.

[0024] The thickeners preferably used are to ensure that the cleaningagent, in the case of inclined or vertical surfaces, does not run off sorapidly and ensures a relatively long contact with the surface. Theseare organic high-molecular-weight substances which absorb liquids,swelling in the process and finally transforming into viscous truesolutions or colloidal dispersions, to increase the viscosity of liquidsand improve the thixotropic properties of gels.

[0025] Further auxiliaries which can be used are interface-activesubstances (surfactants). The surfactants generally fulfil a number ofroles: firstly they improve the wetting of the surfaces to be treated.Especially in the case of structured surfaces, for example floorcoverings or house facades, the cleaner can as a result penetrate betterinto the narrow gaps and cracks. Furthermore, owing to the action ofsurface-active substances, organic compounds, for example oils or fats,to which inorganic dirt particles frequently adhere, can be infiltratedand dispersed. As a result the surface-active substances support theaction of titanium dioxide which breaks down the abovementionedcompounds by oxidation with the result that the dirt can very readily bewashed off from the surface. In addition, the surface-active substancescan bring about formation of a stable foam, as a result of which thecleaner remains at the site of action for a longer period.

[0026] Further preferably usable auxiliaries are dispersants, that is tosay substances which facilitate the dispersion of solid particles in adispersion medium by lowering the surface tension between the twocomponents, that is to say inducing wetting. Dispersants ensure that thesolid constituents of the dispersion, predominantly the semiconductors,in particular titanium dioxide particles, do not sediment, but remain insuspension.

[0027] The aqueous composition comprising a semiconductor can, accordingto the invention, be applied simply and in a targeted manner onto thesurfaces to be treated, preferably by spraying. However, it is alsopossible to apply the cleaning agent using a distributor, for example abrush, a sponge or a cloth.

[0028] It is completely problem-free to remove the applied cleaningagent in a targeted manner, as required, from the desired points, in thesimplest manner by rinsing off with water, if appropriate using ahigh-pressure cleaner. The removal can be facilitated by adding asurface-active substance to the water. It is also possible to remove thecleaning agent dry.

[0029] With respect to the surface to be treated, there are in principleno restrictions, these can be equally smooth or structured, and disposedin the indoor or outdoor area. Particularly preferably, inventivebuildings or parts thereof or outdoor surrounds of buildings can betreated. Further preference is given to the use for cleaning in theindoors field, in particular as kitchen or sanitary cleaner or inmedicine, the pharmaceutical industry or the food industry.

[0030] For the use as a surface cleaner indoors, for example, as akitchen or sanitary cleaner, the cleaning agent is applied and, after acertain time of action, is removed again, for example by wiping orrinsing off.

[0031] In fields where hygiene is critical, such as in medicine, thepharmaceutical industry or food industry, the time of action mustconform with the predetermined reduction factors. Otherwise the useproceeds similarly to that described above for kitchen or sanitarycleaners. The disinfecting and/or cleaning action can be furtherenhanced by exposing the surfaces additionally to UV light.

[0032] The invention also relates to a semiconductor powder, inparticular titanium dioxide, preferably titanium dioxide in the anatasemodification of a mean primary particle size, measured by transmissionelectron microscopy, in the range from 10 to 2000 nm, preferably in therange from 20 to 200 nm, for use as claimed in one of claims 1 to 11.

[0033] Particular preference is also given to a semiconductor powdercontaining at least 60 mol % semiconductor ions and less than 40 mol %of one or more doping ions from subgroup 8 of the Periodic Table of theElements, in particular of rhodium and/or platinum, for use as claimedin one of claims 4 to 11.

[0034] The invention thus has the advantage that any surfaces can becleaned and/or disinfected by simple spraying without any fixing of thesemiconductor in a layer being necessary for this. Activating thesemiconductor material does not require a separate radiation sourcebeing available, daylight, even diffuse daylight, is sufficient forthis. The cleaning agent acts over a relatively long period of severalmonths, and the treatment can be repeated as often as desired after theeffect has decayed. The invention makes it possible to detach inorganicimpurities, since the biological adhesion promoter in the form of extrapolymeric substances of the microorganisms is destroyed or removed. Afurther advantage is that development of resistance does not occur onthe treated surfaces.

[0035] The invention will now be described in more detail with referenceto an example.

[0036] The action of the inventive cleaning agent was established in thefollowing experiments: an aqueous dispersion containing 50 mg oftitanium dioxide in the anatase modification with a mean primaryparticle size of 21 nm (the particle size was determined by transmissionelection microscopy) per liter of dispersion was applied as cleaningagent in the outdoors field to various materials such as wood, washedconcrete slabs, natural rocks, plastics and masonry walls. This aqueouscomposition was applied to said surfaces using a spray bottle not onlywithout further auxiliaries, but also with addition of 0.5% by weight,based on total weight of aqueous dispersion, of an acrylate polymer asadhesion promoter and 5% by weight, based on total weight of aqueousdispersion, of isopropanol as solubilizer. The surfaces were exposedonly in part to the cleaning agent, so that untreated points wereimmediately next to treated points and as a result immediate visualassessment of the cleaning action was possible.

[0037] In the experimental variant using adhesion promoter andsolubilizer, a rapid action was observed, that is to say mosses, lichensand algae died off and disappeared within a few hours. In contrast, thesame action, that is to say death and disappearance of the impuritieswas also observed in the case of treatment with the aqueous compositionwithout auxiliaries, but was delayed in time, that is to say only aftersome days. In both cases the treatment exhibited a long-term action,that is to say even after several months the beneficial patterns ofobservations of the treated surfaces did not change.

1. The use of an aqueous composition comprising a semiconductor at aconcentration of from 5 to 50 mg, based on 1 l of aqueous composition,as cleaning agent for surfaces under the action of light.
 2. The use asclaimed in claim 1, characterized in that the semiconductor is used at aconcentration of from 10 to 50 mg, based on 1 l of aqueous composition.3. The use as claimed in claim 1 or 2, characterized in that thesemiconductor is titanium dioxide, preferably a titanium dioxide whichis at least 70% by weight in the anatase modification.
 4. The use asclaimed in one of claims 1 to 3, characterized by the use of a dopedsemiconductor which contains at least 60 mol % semiconductor ions andless than 40 mol % of one or more doping ions from subgroup 8 of thePeriodic Table of the Elements.
 5. The use as claimed in claim 4,characterized in that a titanium dioxide semiconductor doped withrhodium and/or platinum ions is used.
 6. The use as claimed in one ofclaims 1 to 3 under the action of light of a wavelength in the rangefrom 350 to 400 nm, preferably 380 nm, in the outdoors field, or in theindoors field under the action of artificial light.
 7. The use asclaimed in claim 4 or 5 under the action of light of a wavelength in therange from 400 to 650 nm.
 8. The use as claimed in one of claims 1 to 7,characterized in that the semiconductor has a mean primary particlesize, measured by transmission electron microscopy, in the range from 10to 2000 nm, preferably in the range from 20 to 200 nm.
 9. The use asclaimed in one of claims 1 to 8, characterized in that the aqueouscomposition contains one or more of the following auxiliaries: adhesionpromoters, solubilizers, surface-active substances, dispersants and/orthickeners.
 10. The use as claimed in one of claims 1 to 9,characterized in that the aqueous semiconductor-containing compositionis applied to the surfaces to be cleaned by spraying or by means of adistributor, in particular by means of a brush, a sponge or a cloth. 11.The use as claimed in one of the preceding claims for cleaning buildingsor parts thereof or outdoor surrounds of buildings or for cleaning inthe indoors field, in particular as kitchen or sanitary cleaner or inmedicine, the pharmaceutical industry or the food industry.
 12. Asemiconductor powder, in particular titanium dioxide, preferablytitanium dioxide in the anatase modification, of a mean primary particlesize measured by transmission electron microscopy in the range from 10to 2000 nm, preferably in the range from 20 to 200 nm, for the use asclaimed in one of claims 1 to
 11. 13. A semiconductor powder containingat least 60 mol % semiconductor ions and less than 40 mol % of one ormore doping ions from subgroup 8 of the Periodic Table of the Elements,in particular rhodium and/or platinum, for the use as claimed in one ofclaims 4 to 11.